An electromagnetic flowmeter configured to measure a flow rate of a liquid and a thermometer configured to measure a temperature of a liquid, for example, are known as measurement apparatuses configured to measure a state of a measurement object as a physical amount.
An electromagnetic flowmeter creates a magnetic field inside a measurement pipe by feeding an electric current to a coil. Then, the flow rate of a liquid flowing inside the measurement pipe is measured. Specifically, the flow rate is measured by detecting, by using two electrodes, an electromotive force generated in an orthogonal direction to the magnetic field and being proportional to the flow velocity of the liquid, the electrodes being provided in an inner wall surface of the measurement pipe.
To prevent corrosion of the measurement pipe, the inner surface of the measurement pipe is usually provided with a rubber lining or resin lining using a fluororesin, polyurethane resin, or the like.
There are different structures of the electrodes used in the above-described electromagnetic flowmeter depending on a direction of fitting each electrode into a measurement pipe, namely, an external insertion type configured to fit the electrode by pressing the electrode from the outside of the measurement pipe to the inside of the measurement pipe, and an internal insertion type configured to fit an electrode by pulling the electrode from the inside of the measurement pipe to the outside of the measurement pipe. Normally, the internal insertion type is used because the internal insertion type can readily ensure airtightness even when the pressure inside the measurement pipe is increased.
An electrode structure according to Japanese Examined Utility Model Registration Application Publication No. Hei 3-55865 includes a head portion smaller than a measurement-electrode insertion hole, and a measurement pipe is sealed to prevent leakage of a liquid by clamping a lining material with the head portion and a backup material from the inside of the measurement pipe. The electrode structure of the internal insertion type can suppress variation in distance between the electrodes even when the pressure of a fluid is changed.
Meanwhile, an electrode structure according to Japanese Patent Application Publication No. 2007-240231 is a structure configured to reduce insulation deterioration of an electrode even in the case of using a permeable fluid which permeates a lining body. Accordingly, the electrode structure can exhibit a self-sealing function attributable to a wedge effect and a sealing effect by pressurization to a conic portion of an electrode boss forming an outer periphery of a measurement pipe.
In the electrode structure according to Japanese Patent Application Publication No. 2007-240231, an insulating washer which an electrode penetrates is formed in a size equivalent to the inner diameter of an electrode boss and includes an O ring on an outer peripheral end portion of the washer so as to be tightly sealed with an inner peripheral surface of the electrode boss. The structure allows sealing of the permeable fluid which has permeated the lining body, with the O ring so as to prevent the permeable fluid from reaching a spring or a nut. Thereby, insulation deterioration of the electrode and corrosion of the spring can be reduced.
Meanwhile, a thermometer according to Japanese Patent Application Publication No. 2002-48654 has a structure of inserting a temperature sensor provided with a protective pipe, into a measurement pipe from the outside of the measurement pipe while sealing the protective pipe with the temperature sensor provided therein.
The electrode structure of an electromagnetic flowmeter shown in Japanese Patent Application Publication No. 2007-240231 includes a protruding portion on an outer periphery of an electrode head portion. Accordingly, when the electrode is pulled toward the outside of the measurement pipe, the electrode and the lining body receive a sealing pressure concentrated on the protruding portion. Hence it is also possible to secure sealing even when the lining body is formed of a hard resin such as a fluororesin or hard rubber.
Meanwhile, when the lining body is formed of a material such as a fluororesin which allows permeation of a gas or a vapor, an improvement in airtightness is expected because the electrode structure is configured to seal the electrode boss and the insulating washer with the O ring to prevent leakage of the gas or the vapor which has permeated through the lining body.
However, the surface pressure of the protruding portion on the outer periphery of the electrode is not uniform. When viewed in a cross-sectional direction orthogonal to the axis of the measurement pipe, a gap is formed between an inner peripheral surface of the cylindrical measurement pipe and a flat surface portion under a neck of the electrode head portion excluding the protruding portion, the flat surface portion contacting a lining surface. Hence, the airtightness may be easily impaired. Moreover, the structure is configured to seal the electrode boss and the insulating washer with the O ring and is therefore complicated.
Here, the problems of the sealing structure technique of the electromagnetic flowmeter disclosed in Japanese Patent Application Publication No. 2007-240231 will be described with reference to FIGS. 7A to 8B. The sealing structure cannot achieve uniform surface pressure of the protruding portion on the outer periphery of the electrode.
FIG. 7A shows an x-y cross-sectional view of a measurement pipe 31, the cross-section being parallel to a pipe axis direction x as illustrated in FIG. 7B. Meanwhile, FIG. 8A shows a y-z cross-sectional view of the measurement pipe 31, the cross-section being orthogonal to the pipe axis direction x.
A protruding portion 33c having a semicircular cross-section is provided on an outer peripheral portion of an electrode head portion 30a of an electrode 33, the outer peripheral portion being on an opposite side to a surface contacting a measurement object. In the structure, airtightness is not ensured on a large surface of the electrode but is mainly ensured on a small area of the semicircular surface of the protruding portion. Moreover, the airtightness is ensured by pulling up the electrode with a spring 35 so as to obtain a surface pressure necessary for ensuring the airtightness even when a lining body 32 provided on an inner surface of the measurement pipe 31 is made of a relatively hard resin such as a fluororesin.
Specifically, in the technique, a flat surface is located on the side closer to an electrode shaft portion 33b rather than the protruding portion 33c as shown in FIG. 7A. Therefore, in the cross-section parallel to the pipe axis x of the measurement pipe 31 shown in FIG. 7B, the flat surface closely contacts the lining body 32.
However, in the y-z cross section orthogonal to the pipe axis x as shown in FIG. 8A, the surface of the lining body 32 is rounded. Therefore, the periphery of the protruding portion 33c does not have a uniform surface pressure, and a gap Δg is formed between the flat surface and an inner surface of the cylindrical lining body 32 due to the nonuniform surface pressure. Consequently, the airtightness is likely to be impaired.
The same problem exists in any measurement apparatus including an electrode structure configured to seal a lining body 32 and an electrode 33 by pressing an electrode neck portion against the inner peripheral surface of a cylindrical measurement pipe 31, the electrode neck portion having a flat surface orthogonal to a central axis of an electrode insertion hole.